This invention relates to a vehicle braking system including a braking sub-system comprising; a brake actuator responsive to a brake operating pressure supplied thereto to operate a brake; a supply valve means having a fluid pressure inlet and a brake operating pressure outlet connected to said actuator; a control line connected to the supply valve means to supply a fluid pressure control signal to which the supply valve means is responsive to supply brake operating pressure to the actuator; a pair of solenoid valves in said control line and a managing means responsive to a wheel speed signal to energize the solenoid valves so as to effect one of
a) interruption in the supply of said brake operating pressure to the actuator; PA0 b) interruption in the supply of said brake operating pressure to the actuator and exhaust of the brake operating pressure from the actuator. PA0 a) interruption in the supply of said brake operating pressure to the actuator; PA0 b) interruption in the supply of said brake operating pressure to the actuator and exhaust of the brake operating pressure from the actuator; PA0 a) when de-energized, interrupting supply of the control signal to an operating chamber of the inlet valve and connecting the operating chamber to exhaust to open the inlet valve and permit passage of said brake operating pressure therethrough; PA0 b) when energized, permitting supply of the control signal to said operating chamber of the inlet valve and disconnecting the operating chamber of the inlet valve from exhaust to close the inlet valve and prevent passage of said brake operating pressure therethrough, PA0 a) when de-energized, permitting supply of the control signal to an operating chamber of the exhaust valve and disconnecting the control chamber of the exhaust valve from exhaust to close the exhaust valve and prevent exhaust of said brake operating pressure; PA0 b) when energized, preventing supply of the control signal to the operating chamber of the exhaust valve and connecting the operating chamber of the exhaust valve to exhaust to open the exhaust valve and permit exhaust of said brake operating pressure.
One version of such a system is described in U.S. Pat. No. 4,025,127 wherein the supply valve means comprises a relay valve, said fluid pressure inlet is connected to a uniform fluid pressure source, said control line is connected to a source of pressure responsive to driver demand to provide said control signal and the relay valve is responsive to the control signal to supply brake operating pressure to the actuator from said source, the brake operating pressure being a function of the control signal pressure, and wherein energization of one of said pair of solenoid valves interrupts the control signal supply to the relay valve whilst energization of both of said solenoid valves at the same time is necessary to interrupt the control signal supply to the relay valve and to exhaust the control signal from the relay valve.
Another version of such a system is described in GB-A-1416204 wherein the supply valve means comprises a regulating valve, said fluid pressure inlet is connected to a variable fluid pressure source, the pressure of which is variable under driver demand, said control line is connected to said variable fluid pressure source to provide said control signal and the regulating valve is responsive to the control signal to supply brake operating pressure to the actuator from said source, the brake operating pressure being a function of the pressure of the variable pressure source, and wherein energization one of said pair of solenoid valves interrupts the supply of brake operating pressure to the actuator whilst energization of both of said solenoid valves at the same time is necessary to interrupt the supply of said brake operating pressure to the actuator and to exhaust the brake operating pressure from the actuator.
In both versions the necessity to energize both solenoids at the same time results in an electrical power consumption which is the summation of that used for each solenoid and, where there is more than one such supply valve on a vehicle such as a trailer, this energization at the same time can cause an excessive current consumption. This can be a considerable problem when considering the voltage drop along the length of long vehicles particularly, for example, when one long vehicle is towing another. It is therefore desirable to reduce the amount of current necessary for the efficient operation of the solenoids.
There is a known technique for reducing the amount of current supplied to a solenoid once the solenoid valve has attained its energized position. Whilst such a means is effective to cut down the total amount of current consumption, for a short period of time it is necessary to fully power the solenoid to cause the solenoid valve correctly to attain its energized position. Thus there exists a peak current consumption during the energization of each solenoid valve and the summation of these peaks can still produce an unacceptably high current consumption.
An object of the present invention is to provide a vehicle braking system in which the above mentioned problem is overcome or is reduced.